Precast insulated load bearing roof element and methods of manufacturing a roof element

ABSTRACT

The present disclosure relates to a precast load bearing roof deck element for a building comprising an upper floor layer of concrete, a load bearing steel reinforced lower layer of concrete, at least one thermally insulating layer located between and separating the upper floor layer of concrete and the lower layer of concrete, and a plurality of binders extending between the lower layer of concrete and the upper floor layer of concrete, the roof deck element configured to be mounted on a load bearing construction of the building. The disclosure also relates to a roof construction comprising a load bearing construction, such as a column and beam construction, and at least two of the above mentioned precast load bearing roof deck elements, wherein the roof deck elements are positioned side by side forming a gap between the roof deck elements, wherein the distance between the roof elements is between 0 mm and 100 mm, or between 1 mm and 100 mm. Furthermore the present disclosure relates to a method for manufacturing a load bearing roof deck element and a method for installing a roof construction on a load bearing construction of a building.

The present disclosure relates to a precast load bearing roof elementfor supporting a green roof of a building. The present disclosurefurther relates to a method for manufacturing a load bearing roofelement as well as a method for installing a roof construction on a loadbearing construction of a building.

BACKGROUND OF INVENTION

A green roof, roof garden, living roof or roof landscaping system is aroof of a building that is partially or completely covered withvegetation and a growing medium, planted over a waterproofing membrane.It may also include additional layers such as a root barrier anddrainage and irrigation systems.

There are numerous benefits of green roof systems. Besides the aestheticaspects they offer the potential to address climate change issues suchas increased precipitation. Green roofs can help to reduce CO2 in theair, and subsequently global warming. They may also absorb rainwater andhelp lower urban air temperatures. Green roofs also capture more fineparticles than a smooth standard roof and thus help cleaning the air.This is mainly because of the irregular structure of the surface. Themore irregular the surface, the more fine particles are captured

The main disadvantages of green roofs are that they are technicallycomplicated, labor-intensive and expensive to build and maintain—theinitial costs of installing a green roof can be double that of a normalroof. The additional mass of the soil substrate and retained waterplaces a large strain on the structural support of a building. There arealso high demands on the waterproofing system of the structure, bothbecause water may be retained on the roof and due to the possibility ofroots penetrating the roof membrane and construction. Furthermore,welding of the membrane onsite can also be problematic.

When building a conventional green roof, an insulation layer istypically placed on top of the building structure, which is typicallyconcrete. The insulation is placed on top of the concrete layer on thebuilding. Cutting and attaching the insulation layer on the building isa complicated and time-consuming task. A waterproof membrane is thenmechanically attached on top of the insulation layer. Sometimes thereare several membranes. Cutting, fitting and attaching the membranes iscomplicated and time-consuming and there is a risk that the sometimescomplex geometry can result in leaks. On top of the membranes there aretypically several layers including filters, water retention trays and abarrier against mechanical rupture. On top of this is a light weightsoil for the vegetation. An additional challenge for green roofconstructions is related to efficient drainage during and after heavyraining.

SUMMARY OF INVENTION

In a first embodiment the present disclosure therefore relates to aprecast load bearing roof element for a green roof of a buildingcomprising a load distributing concrete upper floor layer, a loadbearing lower deck layer of concrete, a thermally insulating layerlocated between and separating the upper layer and the lower layer, andpreferably a plurality of binders, preferably independent non-connectedbinders, extending between the lower deck layer and the upper layer. Theroof element is preferably configured to be mounted on (and preferablyspan) a load bearing construction of the building.

This construction addresses the issues related to the difficulties, timeand costs for building a green roof. By manufacturing entire loadbearing roof elements, including an insulation layer, which cansubsequently be mounted on a load bearing construction of a building,much of the work that takes places on the roof during buildingconstruction can be moved to the ground or to a factory. The insulationlayer is built into the roof element and is sandwiched between twolayers of concrete. Therefore the roof elements can be said to beself-insulating. The presently disclosed roof elements are strong andrigid enough to serve as platform for a green roof and can betransported in entire precast pieces that can be placed directly on aload bearing construction of a building. Load bearing in the context ofthe presently disclosed precast load bearing roof element refers to theroof element being able to resist the load of a green roof.

Preferably, the roof element is capable of carrying the green roofincluding soil soaked with water, live load, precipitation, e.g. snow,the weight of the roof element itself and be able to span the distancebetween load bearing constructions of the building. In order to functionas a green roof (or terrace) the upper floor layer must be able to carrythe weight of precipitation (e.g. snow), soil, possibly soaked in water,and live load, i.e. people walking and jumping on the upper floor layer.The weight bearing requirements of a green roof is thereforesubstantially higher than for a normal roof. In one embodiment thepresently disclosed roof element is therefore configured such that theupper floor layer of concrete can carry at least 500 kg/m², morepreferably at least 600 kg/m², even more preferably least 700 kg/m², yetmore preferably at least 800 kg/m², most preferably at least 900 kg/m².

The main purpose of the upper floor layer of concrete is distributingthe load through the insulation layer to the lower deck layer, which isthe load bearing part. The lower deck layer is therefore preferablyreinforced with substantially horizontal steel reinforcement bars. Theinsulating layer separates the upper layer and lower layer, and theinsulating layer is preferably made of a substantially rigid material asexplained below and configured to transfer the load from the upper floorlayer to the lower deck layer. The solution with an inner insulationlayer and surrounding layers of concrete is also robust and relativelycheap to manufacture. In a preferred embodiment, the roof elementcomprises a plurality of binders extending between the lower layer andthe upper layer. Preferably the binders are cast into the lower layerand upper layer, i.e. upper floor layer and lower deck layer. In oneembodiment the binders comprise at least one slanted steel rod. Slantedin this context can be understood as the at least one binder extendingboth in a vertical direction between the upper layer and the lower layerand in a horizontal direction. This has several advantages; it preventssideways movement of the upper layer in relation to the lower layer andat the same it limits the thermal bridge between the upper and lowerlayer since the fact that the binders extend also in the horizontaldirection makes the path between the lower and upper layers of concretelonger than if the binders would only extend in the vertical direction.

The binders may be slanted rods of steel, stainless steel or galvanizedsteel or other suitable materials. Preferably, the slanted steel rods donot carry any vertical load of the upper layer of concrete. The bindersare therefore preferably independent, e.g. they are not connected toeach other, unlike e.g. latticeworks which are interconnected lattices.The binders of the present disclosure are configured to prevent sidewaysmovement of the upper layer and preferably flexes slightly downwardswhen exposed to additional load from the upper layer.

In one embodiment, the plurality of binders extend through the at leastone thermally insulating layer, preferably limiting the thermal bridgeto the contact between the layers of concrete and the binders. Thebinders are relatively thin and therefore the thermal bridge between theupper and lower layers of concrete is negligible or substantiallynegligible.

In one embodiment, the lower layer of concrete is pre-tensionedconcrete. Preferably, the lower layer of concrete has embedded,substantially horizontal reinforcement bars as shown in e.g. FIG. 6(reinforcement steel bars 27, steel reinforcement bars also shown ine.g. FIGS. 9-14). The inventor has realized that by dimensioning lowerdeck layer of concrete appropriately and including reinforcement bars inthe lower layer of concrete and combining it with binders, the deck isload bearing and at the same time thermally efficient, while it is alsopossible to manufacture and mount the element efficiently.

A further aspect of the invention relates to the roof element having awaterproof membrane attached, possibly welded, on the upper surface ofthe upper floor layer of concrete. Attaching this membrane is a ratherexpensive and time-consuming process when performed on-site. It requiresprofessionals to go to the building and bring tools and materials. Inthe present invention the membrane may be attached, for example welded,on the roof elements in a factory under more optimal manufacturingconditions. Furthermore the actual construction time for the building isshortened since the roof elements can be delivered in a state ready tobe placed directly on a load bearing construction of a building.

Preferably, the insulating layer is made of a rigid and light material,such as polyisocyanurate (PIR), polyurethane (PUR) or expandedpolystyrene (EPS). As an example, a layer of PIR may bear a weight of2000 kg/m². Therefore, the insulating layer may bear the weight of theupper layer and above layers (upper layer of concrete, soil etc.)Various shapes of the layers are possible—in one embodiment the lowerdeck layer of concrete is load bearing and shaped as a rectangularcontainer with five closed sides, wherein the upper side is open. Theload bearing, steel reinforced lower deck layer of concrete may be castaround an inner volume of insulation, this inner volume of insulationthereby at least partly forming the lower deck layer.

The present invention also relates to a roof construction comprising aload bearing construction, such as a column and beam construction orsimply load bearing walls of a building, and a number of theabove-mentioned load bearing roof elements. The roof elements arepositioned side by side, possibly forming a gap between the roofelements, in which concrete and/or autoclaved aerated concrete isfilled. An additional waterproof membrane may be added to cover the gapsalong with other elements to complete the roof for use as e.g. a greenroof. In one embodiment, the roof construction comprises one precastload bearing roof element. Such a construction may form a terrace, forexample an outdoor space adjacent to an apartment.

Another aspect of the invention relates to a method for manufacturing aload bearing roof element, in which the layers of the roof element arecast and assembled, and a waterproof membrane is attached, possiblywelded, on the upper surface of the upper layer of concrete, such thatthe membrane covers at least the entire upper surface of the roofelement. The advantage of this method is that all steps can be performedon the ground instead of on-site, possibly in a factory, and a roofelement is obtained, which can be lifted and placed side by side withother load bearing roof elements directly on a load bearing constructionof a building as it is.

DESCRIPTION OF DRAWINGS

The invention will in the following be described in greater detail withreference to the drawings. The drawings are exemplary and are intendedto illustrate some of the features of the present method and unit andare not to be construed as limiting to the presently disclosedinvention.

Prior Art

FIG. 1a shows a cross section of a prior art green roof solution withscreed and insulating layer on top of a concrete deck.

FIG. 1b shows a cross section of the whole green roof solution of FIG. 1a.

FIG. 2 shows an alternative typical prior art solution of a concretehollow core deck.

Various Embodiments of the Present Invention

FIG. 3a shows a cross section of a load bearing roof element mounted ona load bearing construction.

FIG. 3b shows a cross section of the whole load bearing roof element ofFIG. 3 a.

FIG. 4a shows a load bearing roof element with a visible second portionof a thermally insulating layer for forming the slope of the roofelement comprising a rigid and light material.

FIG. 4b shows the load bearing roof element of FIG. 4a with a waterproofmembrane attached to the upper surface of the upper layer.

FIG. 4c shows a cross section of the load bearing roof element of FIG.4b with a first portion of a thermally insulating layer comprisingbuilding insulation material, and a second portion of a thermallyinsulating layer comprising a rigid and light material.

FIG. 5a shows a roof construction comprising a load bearing constructionand four load bearing roof elements.

FIG. 5b shows the roof construction of FIG. 5a with additionalwaterproof membranes overlapping two neighboring roof elements, therebycovering the gap between the roof elements, and additional insulationelements (7).

FIG. 5c shows the roof construction of FIGS. 5a and 5b with a layer ofmembrane 8 covering the additional insulation elements.

FIG. 6 shows a cross section of two neighboring roof elements of a roofconstruction with a gap between the elements filled with concrete andautoclaved aerated concrete, the roof elements of the roof constructionhaving binders configured to stabilize the upper layer of concrete roofsideways.

FIG. 7 shows the lower layer of a load bearing roof element shaped as arectangular container.

FIG. 8a shows a first portion of a thermally insulating layer andbinders configured to stabilize the upper layer roof element sideways inrelation to the lower layer of concrete.

FIG. 8b shows the first portion of a thermally insulating layer andbinders of FIG. 8a and a second layer of the first portion of athermally insulating layer comprising a rigid and light material

FIG. 8c shows the second portion and binders of FIG. 8b with a secondportion of a thermally insulation layer comprising a rigid and lightmaterial.

FIG. 8d shows the roof element of FIGS. 8a-c having an upper layer ofconcrete.

FIG. 9 shows a cross section of another embodiment of a roof elementhaving binders in the form of steel rods.

FIG. 10 shows a further embodiment of a roof element having a binderconfigured to stabilize the upper layer of the roof element sideways,the binder having a middle part extending both vertically (extendingbetween the upper and lower layers of concrete) and horizontally.

FIG. 11 shows another embodiment having pairs of binders forming anX-like structure.

FIG. 12 shows a cross section another embodiment of a roof element.

FIG. 13 shows a further embodiment of the presently disclosed precastload bearing roof element.

FIG. 14 shows an embodiment of the presently disclosed roof constructionhaving two roof elements placed side by side, forming a terrace/greenroof.

FIG. 15 shows a cross section of two neighboring roof elements of a roofconstruction, the roof elements being bolted together with brackets.

FIG. 16 shows an embodiment of a roof element having brackets cast intothe lower layer of concrete.

DETAILED DESCRIPTION OF THE INVENTION

One purpose of the present invention is to provide a robust, simple andcost efficient roof element for a building. The roof element may be aroof slab for a green roof. Therefore the presently disclosed inventionrelates to a precast load bearing roof element for a building comprisingan upper layer of concrete, a lower layer of concrete, and at least onethermally insulating layer between the upper layer and the lower layer,the roof element configured to be mounted on a load bearing constructionof the building. As stated this is also a solution that allows the roofelements to be manufactured in e.g. a factory rather than on-site.

In a preferred embodiment the roof elements have a substantially planeshape and/or a substantially plane upper surface of the upper layer. Theroof elements are typically rectangular but may have other geometricalshapes to fit different types of load bearing constructions. In oneembodiment, the roof element comprises a plurality of binders extendingbetween the lower layer and the upper layer of concrete. Preferably thebinders are cast into the lower layer and upper layer. In oneembodiment, the binders comprise at least one slanted steel rod. Slantedin this context can be understood as the binders extending both in avertical direction between the upper layer and the lower layer and in ahorizontal direction. This has several advantages; it prevents sidewaysmovement of the upper layer in relation to the lower layer and at thesame time the geometry limits the thermal bridge between the upper andlower layer since the fact that the binders extend also in thehorizontal direction makes the path between the lower and upper layersof concrete longer than if the binders would only extend in the verticaldirection. The binders may be slanted rods of steel, stainless steel orgalvanized steel or other suitable materials. A binder may be an elementof metal which spans between two layers of concrete and binds the twolayers together.

The binders may have a lower part, a middle part, and an upper part,wherein the lower and upper parts may be cast into the concrete layers.The binders have the task of stabilizing the upper layer of the roofelement in relation to the lower layer of the roof element, preventingsideways movement of the layers in relation to each other. There aredifferent possible configurations and shapes of the binders. In oneembodiment, the lower and upper parts are substantially horizontal,whereas the middle part extends both in the vertical and horizontaldirection. This embodiment is shown in FIG. 10. The shape of the bindercan be said to be that of a sloping ‘Z’. In another embodiment, thelower and upper parts are substantially vertical, as shown in e.g. FIG.12. In one embodiment the diameter of the at least one binder is atleast 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or atleast 6 mm, or at least 7 mm, or at least 8 mm, or at least 9 mm, or atleast 10 mm, or at least 15 mm, or at least 20 mm. The binder(s) mayalso be shaped as plates, wherein the cross section of the plate has theshapes as describes above i.e. either Z-shaped or having substantiallyvertically lower and upper parts and a middle parts extending both inthe vertical and horizontal direction. Preferably, the upper and lowerparts of the binders are cast into the precast load bearing roof elementas part of the manufacturing. A reinforced steel net may be included,around which the top layer of concrete is poured, preferably placed ontop of the at least one thermally insulation layer, connected to thebinders.

The binders are preferably distributed substantially equally across thehorizontal area of the roof element, preferably with some distance inthe horizontal plane between the binders such that they are independentbinders. The density of binders, i.e. the number of binders per areaunit or length unit of the roof elements, is dependent on the dimensionsof the upper layer of concrete and the strain between the upper layer ofconcrete and lower layer of concrete. In one embodiment, there are 2-5binders/m², or 2-5 binders/m², or at least 2 binders/m², or at least 3binders/m², or at least 4 binders/m², or at least 5 binders/m².

Preferably, the binders are independent i.e. not connected internally,thereby limiting the thermal bridge between the upper and lower layersof concrete. Independent non-connected binders ensure that the bindersdo not carry any weight

The lower layer of concrete may be load bearing, whereas the upper layerof concrete may distribute the weight on the roof element to the lowerlayer of concrete.

In existing green roof technology there is typically a lower layer ofconcrete, which is part of the building. Sometimes there is also ascreed on top of the concrete, which can be described as a thin layer ofconcrete poured on-site on top of the structural concrete. On top of thescreed there is then a layer of insulation and one or more membrane(s).FIGS. 1a and 1b show an example of a prior art green roof solution asdescribed. In the presently disclosed invention the roof elements can bedelivered as entire pieces. FIGS. 3a, 3b, 4a, 4b, and 4c are examples ofroof elements according to the present invention. Since the compositepieces of the present invention do not have to be assembled on-site,there are not only the advantages of manufacturing the roof elements ina more controlled environment; there are also better possibilities fortesting the quality of the manufactured pieces, and thus reducing therisk of leakages. Because the insulation is incorporated into the deckelement, and because the membrane is entirely welded to the top layer ofconcrete, the resulting deck element reduces work on the building siteand reduces the possibility for human error, especially regarding themembrane's water proofing function.

The roof elements are typically, but not necessarily, mountedhorizontally on a load bearing construction of the building, for exampleon a column and beam construction. A green roof may also be slightlysloped. Therefore, the roof elements of the present invention may alsobe mounted such that the roof element is sloped less than 10°, or lessthan 11°, or less than 12°, or less than 13°, or less than 14°, or lessthan 15°, or less than 20°, or less than 25° in the longitudinaldirection of the roof element in relation to a horizontal line.

A person skilled in the art will understand that the presently disclosedtechnology may have additional embodiments and that the technology maybe practiced without the exact disclosure of all imaginable embodiments.

Insulation Layer

In one embodiment of the present invention the thermally insulatinglayer(s) separate(s) the upper layer of concrete from the lower layer ofconcrete to reduce the transfer of thermal energy between the twolayers. In the typical use of the roof element the lower layer isexposed to room temperature and the upper layer is exposed to outdoortemperate i.e. varying temperature. Low thermal conductivity (k)materials reduce heat fluxes. Preferably, the thermally insulating(s)layer should have low thermal conductivity. Therefore, in one embodimentof the present invention one portion of the thermally insulating layeris selected from the group of polystyrene, polyisocyanurate (PIR),polyurethane (PUR), cellular glass, wood fiber and the like insulatingmaterials with similar compressive strength. Other candidates forinsulation material could be cellulose, glass wool, rock wool, urethanefoam, vermiculite, perlite, plant fiber, recycled cotton denim, plantstraw, and animal fiber. However, these would probably not have asuitable compressive strength for the purpose of carrying a green roof.

Another aspect of the present invention relates to at least one secondportion of the thermally insulating layer(s) comprising a rigid andlight material, such as autoclaved aerated concrete. The purpose of thisrigid, light and thermally insulating layer is to reduce the impact ofthe weight of the upper layer and other layers on top of the upper layeron the insulation layer. Therefore, in one embodiment the second portioncan be said to form a weight bearing connection between the upper andlower layers of concrete.

In one embodiment, the at least one thermally insulating layer comprisesat least one second portion on top of the first portion comprising arigid and light material such as polyisocyanurate (PIR) and polyurethane(PUR), the second portion configured to form a slope of the upper layerin relation to the lower layer. The second portion can form astair-shaped slope of insulating material as shown in FIG. 5a (secondportion 5). Alternatively, the slope is formed by cutting the secondportion of insulating material to a smoothly sloping area. The concretefloor upper layer and the lower deck layer form an inner volume therebetween. Preferably, the inner volume is non-ventilated and preferablythe inner volume is sealed such that that there is no connection of e.g.moisture from the outside. Preferably, the thermally insulating layerfills this inner volume substantially, most preferably completely fillsthis inner volume between the upper floor layer and the lower decklayer. This is one way of creating a non-ventilated inner volumeinsulating layer between the upper floor layer and the lower deck layerthat can carry the weight on a green roof, i.e. transfer the weight ontop of the top floor layer to the load carrying lower deck layer.

Shapes, Sizes

As stated the load bearing roof elements according to the presentinvention can be considered to constitute slabs or decks on top of thecolumn and beam construction or load bearing walls. This means that thelower layer of concrete needs to be able to resist the verticalgravitational force of itself and the layers on top of it. At the sametime the lower layer of concrete is preferably designed such that itreduces the thermal conductivity between the upper layer and the lowerlayer. In one embodiment the lower layer is load bearing and shaped as arectangular container with five closed sides, and wherein the upper sideis open. FIG. 7 shows an example of a lower layer according to thepresent layer shaped as a rectangular container with five closed sides,and wherein the upper side is open. The container in FIG. 7 also has oneinner wall of concrete. The thermally insulating layer preferably fillsthe inner volume of the lower layer shaped as rectangular container(s).This shape and configuration may be useful to provide thick insulationin some areas while a strong construction of the element is maintained.The container-shaped load bearing steel reinforced lower deck layer ofconcrete may, in the context of the presently disclosed roof element forgreen roofs, have the function that the load is carried by the upwardlyextending edge portions (or middle portions) of the container while theinterior mainly serves as thermal insulation, which also reduces theweight. A shape as shown in e.g. FIGS. 12-13 is efficient in terms ofweight and substantially as load bearing as a design not having therecesses. In one embodiment, the thickness of the upwardly extendingportions in FIG. 12-13 is at least 250 mm, or at least 300 mm, while thethickness of the thinner middle portions are 100-200 mm, or 120-160 mm,such as 130 mm, or 140 mm or 150 mm. The upwardly extending portions maybe referred to as compression zones—preferably the compression zones areconfigured to bear the load of the green roof in coordination with thereinforcing steel bars in the lower deck layer.

The thickness of the upper layer of concrete should be chosen such thatit provides a good protection for the insulation layer from the layersabove, e.g. soil, in terms of weight and moisture, and contributes toholding the roof element together. In one embodiment the thickness ofthe upper layer is in the range of 50 mm and 250 mm, such as in therange of 50 mm and 100 mm, or such as in the range of 100 mm and 150 mm,or such as in the range of 150 mm and 200 mm, or such as in the range of150 mm and 250 mm, for example 50 mm, or 55 mm, or 60 mm, or 70 mm, or80 mm, or 90 mm, or 100 mm, or 150 mm, or 200 mm, or 250 mm.

As stated the lower layer of concrete needs to be able to resist thevertical gravitational force of itself and the layers on top of it. Inone embodiment the thickness of the lower layer is in the range of 50 mmand 250 mm, such as in the range of 50 mm and 100 mm, or such as in therange of 100 mm and 150 mm, or such as in the range of 150 mm and 200mm, or such as in the range of 150 mm and 250 mm, for example 50 mm, or55 mm, or 60 mm, or 70 mm, or 80 mm, or 90 mm, or 100 mm, or 150 mm, or200 mm, or 250 mm, or 300 mm, or 400 mm, or 500 mm, or 600 mm. Thethickness of the lower layer of concrete has a proportional relationshipto the distance being spanned by the deck element. For example, a roofelement of approximately 8.0 meters could have a lower layer of 300-400mm depending on the load.

The thickness of the thermally insulating layer depends on a number ofparameters, such as expected temperature differences between the upperand lower layer, how rigid the insulation layer itself is, propertiesand volume of a second rigid portion supporting the structure of theinsulating layer etc. In one embodiment the thickness of the thermallyinsulating layer is in the range of 50 mm and 300 mm, such as in therange of 50 mm and 100 mm, or such as in the range of 100 mm and 150 mm,or such as in the range of 150 mm and 200 mm, or such as in the range of200 mm and 300 mm, for example 50 mm, or 55 mm, or 60 mm, or 70 mm, or80 mm, or 90 mm, or 100 mm, or 150 mm, or 200 mm, or 250 mm, or 300 mm,or 400 mm, or 500 mm, or 600 mm, or 700 mm, or 800 mm, or 900 mm.

The combined thickness of the upper layer, the lower layer and the atleast one thermally insulating layer represents one of the roof elementsouter dimensions. As one of the goals with the presently disclosedinvention is that the roof elements should be easy to transport andmount, preferably the elements should not be too heavy. This also savesmaterial. On the other hand, the roof elements should be robust and loadbearing. In one embodiment the combined thickness of the upper layer,the lower layer and the at least one thermally insulating layer is inthe range of 300 mm and 600 mm, such as in the range of 300 mm and 400mm, or such as in the range of 400 mm and 500 mm, or such as in therange of 400 mm and 600 mm, or such as in the range of 300 mm and 350mm, or such as in the range of 350 mm and 400 mm, or such as in therange of 400 mm and 450 mm, or such as in the range of 450 mm and 500mm, for example 300 mm, or 310 mm, or 320 mm, or 330 mm, or 340 mm, or350 mm, or 400 mm, or 450 mm, or 500 mm, or 550 mm, or 600 mm, or 700mm, or 800 mm, or 900 mm, or 1000 mm.

The length of the roof element is a matter of how much weight andtension the layers support. A longer deck element, which only rests onan existing load bearing construction at the ends, is exposed to greatergravitational forces than a short deck element. Therefore, the length ofthe deck element has to be adapted to other choices that are made. Inone embodiment the length of the roof element is in the range of 4meters and 10 meters, such as in the range of 4 meters and 7 meters, orsuch as in the range of 5 meters and 10 meters, for example 4 meters, or4.5 meters, or 5.0 meters or 5.5 meters, or 6.0 meters, or 7.0 meters,or 8.0 meters, or 9.0 meters, or 10.0 meters, or 11.0 meters, or 12.0meters.

The width of the roof element is relatively open in the scope of thepresently disclosed invention. Preferably the width is limited toapproximately 3.0 meters to avoid that the roof elements becomeponderous to move. On the other hand, too narrow roof elements willrequire more connecting surfaces to seal on a roof constructionincluding a number of roof elements. A narrow roof element will alsorequire more lifts by crane. A typical standard width of ordinary roofelements is 1.2 meters. However a width of 2.4 meters is also standardand would also be possible. In one embodiment the width of the roofelement is in the range of 0.5 meters and 3 meters, such as in the rangeof 0.5 meters and 1.5 meters, or such as in the range of 1.5 meters and2.5 meters, or such as in the range of 2.5 meters and 3.0 meters, orsuch as in the range of 1.0 meters and 1.4 meters mm, or such as in therange of 2.4 meters and 2.8 meters, for example 0.5 meters, or 0.6meters, or 0.7 meters or 0.8 meters, or 0.9 meters, or 1.0 meters, or1.2 meters, or 1.4 meters, or 1.6 meters, or 2.0 meters, or 2.4 meters,or 2.8 meters, or 3.0 meters.

A further aspect of the present invention relates to the upper layer ofthe roof element being slightly convex exteriorly. For some buildings,climates and/or roof vegetation it may be good to have a slightly convexroof to lead away some of the rainwater that reaches the membrane. Inone embodiment the height difference between the highest point and thelowest point of the outer surface of the convex upper layer is less than100 mm, or less than 90 mm, or less than 80 mm, or less than 70 mm, orless than 60 mm, or less than 50 mm. An alternative to having anexteriorly convex upper layer of the roof element is to mount theelement such that it has a slope of approximately between 1:40 and 1:80(height difference: length) to ensure water movement towards a drain.The roof deck element may have an embedded drain as shown in e.g. FIGS.5a-c . The movement of water on the membrane can also be achieved bysloping the entire upper layer of concrete in relation to the lowerlayer of concrete. Slope can also be achieved by sloping the entireconcrete element, so that one end of the element is higher than theother.

Membrane

A further aspect of the invention relates to the roof element having awaterproof membrane attached, possibly welded, on the upper surface ofthe upper layer. Attaching this membrane is a rather expensive andtime-consuming process when performed on-site. The inventor has realizedthat by attaching the membrane to the roof element as part of theprocess of building the element, a safer, more robust and cheaperproduct can be achieved, because the membrane then becomes part of thepresently disclosed precast roof element. Furthermore the actualconstruction time for the building may be shortened since the roofelements can be delivered in a state ready to be placed directly on aload bearing construction of a building. The membrane may cover theentire upper surface of the upper layer.

The membrane can be of one or several materials selected from the groupof synthetic rubber and/or thermoplastic and/or modified bitumen, and/orpolyurethane and/or metal, and/or roofing felt. Preferably the membraneof a roof element is seamless, meaning that it consists of only onepiece of membrane. Where the concrete element is wider than a standardwidth of membrane, the membrane can be covered with two or more pieces,where one piece laps over the other at their meeting point/intersection.If desirable the membrane may also be configured to lead water away fromthe roof element and towards a drain.

If the roof element is used for a green roof there may be roots from thevegetation, which could puncture a conventional green roof constructionand cause leaks and decay. The roof elements according to the presentinvention are generally more resistant against growing roots since theinsulation layer is protected inside an upper and lower layer ofconcrete. However, a root repellant membrane further increases theresistance against growing roots. The roof element may further comprisean additional root repellent membrane for this purpose.

Roof Construction

A further aspect of the invention relates to a roof constructioncomprising a load bearing construction, such as a column and beamconstruction or load bearing walls, and at least two of the abovementioned precast load bearing roof elements, wherein the roof elementsare positioned side by side forming a gap between the roof elements,wherein the distance between the roof elements is between 0 mm and 100mm, or between 1 mm and 100 mm, or between 1 mm and 10 mm, or between 1mm and 20 mm, or between 1 mm and 30 mm, or between 1 mm and 40 mm, orbetween 1 mm and 50 mm, or between 0 mm and 10 mm, or between 0 mm and20 mm, or between 0 mm and 30 mm, or between 0 mm and 40 mm, or between0 mm and 50 mm. Such a roof takes advantage of the simplicity androbustness of the roof elements as described above.

The gaps between the roof elements may be filled with concrete and/orautoclaved aerated concrete, as shown in FIG. 6. Preferably the concretein the gaps is configured to resist diaphragm actions of the roofelements. There may be tension, compression and movements of the roofelements. The concrete is better suited for resisting these physicalimpacts than autoclaved aerated concrete; however, the autoclavedaerated concrete has better insulating properties and is lighter. Theinventor has realized that by having a first layer of concrete in thegaps to stabilize the construction and a second layer of autoclavedaerated concrete to insulate the roof, the construction benefits fromboth materials.

The roof elements preferably have a waterproof membrane attached whendelivered to the building site. However, the gaps between the roofelements have to be covered with additional strips of waterproofmembrane to seal the whole roof construction. Therefore, the roofconstruction may further comprise additional waterproof membranesoverlapping two neighboring roof elements, thereby covering the gapbetween the roof elements. In one embodiment of the present invention,the additional waterproof membrane is welded on the upper surfaces ofthe two neighboring roof elements. The additional waterproof membraneshould be sufficiently wide to cover the gaps between the roof elements.The width of the additional waterproof membrane is in the range of 30 mmand 400 mm, such as in the range of 30 mm and 100 mm, or such as in therange of 100 mm and 200 mm, or such as in the range of 200 mm and 300mm, or such as in the range of 300 mm and 400 mm, or such as in therange of 100 mm and 300 mm, for example 30 mm, or 40 mm, or 50 mm or 60mm, or 70 mm, or 80 mm, or 90 mm, or 100 mm, or 120 mm, or 140 mm, or160 mm, or 180 mm, or 200 mm, or 250 mm, or 300 mm, or 350 mm, or 400mm.

Since the thermally insulating layer(s) may comprise a rigid and lightmaterial, such as autoclaved aerated concrete, to reduce the impact ofthe weight of the upper layer and other layers on top of the upper layeron the insulation layer, there may be sections of the roof element thatcould be considered to be more exposed to thermal conductivity betweenthe upper layer of concrete and the lower layer of concrete. One aspectof the present invention relates to the roof construction furthercomprising additional insulation elements on the upper side of the upperlayer, wherein the additional insulation elements cover at least a partof the vertical extensions of the sections of concrete and/or autoclavedaerated concrete. The advantage of using additional insulation elementson the upper side of the upper layer of concrete in the presentinvention is that if some areas of the roof elements are less insulatedthan others, the additional insulation elements may compensate in theseareas. Additional insulation elements (7) are shown in e.g. FIG. 5b andFIG. 12. In FIG. 12, the additional insulation elements have a crosssection that is substantially triangular. In the example of FIG. 12 itis shown how the additional insulation elements counteracts the thermalbridges on the edges of the roof element.

The thickness of the additional insulation elements is in the range of50 mm and 200 mm, such as in the range of 50 mm and 100 mm, or such asin the range of 100 mm and 150 mm, or such as in the range of 150 mm and200 mm, for example 50 mm, or 55 mm, or 60 mm or 65 mm, or 70 mm, or 80mm, or 90 mm, or 100 mm, or 120 mm, or 140 mm, or 160 mm, or 180 mm, or200 mm. In one embodiment the sides of the additional insulation towardsthe center of the roof elements are sloped. Examples of such additionalinsulation elements are shown in FIG. 5b (additional insulation elements7). The cross section of these elements may be substantially triangular.To protect the additional insulation elements against moisture and/orexternal physical/mechanical impact, and/or to protect the membrane, alayer of metal, for example steel, may be placed to cover the additionalinsulation elements.

Methods for Manufacturing and Installation

The present invention addresses the issues related to the difficulties,time and costs for building a green roof. Besides the load bearing roofelement itself, the present invention relates to a method formanufacturing a load bearing roof element, comprising the steps: castinga lower layer of concrete around an inner volume of insulation; adding alayer of thermally insulating material on the lower layer of concrete;

casting a lower end of at least one binder into the lower layer ofconcrete; casting an upper layer of concrete on the layer of thermallyinsulating material and casting an upper end of the at least one binderinto the upper layer of concrete; attaching a waterproof membrane on theupper surface of the upper layer of concrete, such that the membranecovers at least the entire upper surface of the roof element or theentire upper surface except strips of surface at the edges of thesurface, such as strips having a width of less than 50 mm, or less than100 mm, or less than 150 mm from the edges.

Another embodiment of the manufacturing method relates for manufacturinga load bearing roof element, comprising the steps of casting a lowerdeck layer of concrete around an inner volume of insulation and around areinforcing steel bars, preferably pre-tensioned reinforcing steel bars;casting a lower end of a plurality of independent binders into the uppersurface of the lower deck layer of concrete; adding a layer of thermallyinsulating material on the lower deck layer of concrete; casting anupper floor layer of concrete on the layer of thermally insulatingmaterial and casting an upper end of said binders into the lower surfaceof the upper floor layer of concrete; and attaching a waterproofmembrane on the upper surface of the upper floor layer of concrete, suchthat the membrane covers at least the entire upper floor surface of theroof element, wherein the membrane is welded on the upper surface of theupper floor layer of concrete.

These processes can be performed in for example a factory rather thanthe constituent parts of the load bearing roof element being assembledon the roof, which is clearly an advantage. The process is simpler thanthe existing processes that are typically used for building a greenroof, which requires that all of the steps are performed on-site, on thebuilding. Attaching a membrane on the insulation in conventional greenroof constructions is typically a complicated and time-consuming task.The inventor has realized that a simpler solution may be to weld themembrane on the upper surface of the upper layer of concrete of the roofelement of the present invention. This can be performed as part of themanufacturing of the roof element and with standard sizes of the roofelements standard sizes of the membrane can also be used.

A further aspect of the invention relates to a method for installing aroof construction on a load bearing construction of a building,comprising the steps: manufacturing a number of thermally insulated loadbearing roof elements according to the method described above; liftingthe thermally insulated load bearing roof elements and placing them onthe load bearing construction side by side, thereby forming gaps betweenthe roof elements, wherein the distances between the roof elements arebetween 0 mm and 100 mm, or between 1 mm and 100 mm, or between 1 mm and10 mm, or between 1 mm and 20 mm, or between 1 mm and 30 mm, or between1 mm and 40 mm, or between 1 mm and 50 mm, or between 0 mm and 10 mm, orbetween 0 mm and 20 mm, or between 0 mm and 30 mm, or between 0 mm and40 mm, or between 0 mm and 50 mm; filling the gaps with concrete and/orautoclaved aerated concrete. By using roof elements that come in piecesready to be placed on the load bearing construction side by side, timecan be saved on the building site.

After the roof elements have been placed on the load bearingconstruction and the gaps between the elements have been filled withconcrete and/or autoclaved aerated concrete, additional waterproofmembranes overlapping two neighboring roof elements may be welded,thereby covering the gap between the roof elements.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1a shows a cross section of a prior art green roof solution withscreed and insulating layer on top of a concrete deck. A concrete roofelement 101 is mounted on a load bearing construction of a building. Ontop of the concrete deck there is a screed 102 that has been pouredon-site on top of the concrete. On top of the screed 102 there is amembrane 103, an insulating layer 104 and an additional two-layermembrane 105. Cutting, placing and attaching the membranes andinsulating layer have to be done on-site in this solution. 106 refers toa water reservoir of the prior art green roof. FIG. 1b shows a crosssection of the whole green roof solution of FIG. 1 a.

FIG. 2 shows an alternative prior art solution of a concrete deck. Thesolution comprises two slabs 201, wherein the gap between the slabs isfilled with concrete 202. The gap also has steel reinforcements 203. Ontop of the insulating slabs 201 there is a screed 204 that has beenpoured on-site on top of the concrete, and a membrane 205.

FIG. 3a shows a cross section of a load bearing roof element 1 mountedon a load bearing construction. The roof element has an upper layer ofconcrete 2, a lower layer of concrete 3, with a first portion 4 of athermally insulating layer comprising building insulation material, anda second portion 5 of a thermally insulating layer, and a waterproofmembrane 6. The figure also shows an additional insulation element 7.The membrane 6 runs up the additional insulation element 7 and is tuckedinto a part of the load bearing construction. The additional insulationelement 7 is covered by a layer of metal 8. The figure also shows thegrowing medium 9 (soil) of the green roof and the steel construction 10of the load bearing construction. FIG. 3b shows a cross section of thewhole load bearing roof element of FIG. 3a with vegetation on the greenroof.

FIG. 4a shows a load bearing roof element 1 with a second portion 5 of athermally insulating layer for forming the slope of the roof elementcomprising a rigid and light material, an upper layer of concrete 2, alower layer of concrete 3. Inside this roof element there is typically afirst portion of a thermally insulating layer comprising buildinginsulation material. FIG. 4b shows the load bearing roof element of FIG.4a with a waterproof membrane 6 attached on the upper surface of theupper layer. FIG. 4c shows a cross section of the load bearing roofelement of FIG. 4b with a first portion 4 of a thermally insulatinglayer comprising building insulation material, and a second portion 5 ofa thermally insulating layer. In this figure the second portion 5 ofrigid and light material forms the slope of the roof element. The upperlayer 2 may become slightly convex exteriorly due to camber changes.

FIG. 5a shows a roof construction 11 comprising a load bearingconstruction and four load bearing roof elements, the roof construction11, including additional insulation elements 7 (shown in FIG. 5c ). Eachload bearing roof element has a visible upper layer of concrete 2, alower layer of concrete 3, and a waterproof membrane 6. The roofconstruction in this example is a bearing wall construction havingconcrete walls 13. FIG. 5b shows the roof construction of FIG. 5a withadditional waterproof membranes 14 overlapping two neighboring roofelements, thereby covering the gap between the roof elements. Thismembrane may be welded on the roof elements. FIG. 5c shows the roofconstruction of FIG. 5b with layers of membrane covering the membrane 6that is welded to the deck elements and the additional insulationelements 7.

FIG. 6 shows a cross section of two neighboring roof elements 1 of aroof construction with a gap between the elements filled with concreteor autoclaved aerated concrete 15. Each roof element in this figure hasan upper layer of concrete 2, a lower layer of concrete 3, a with afirst portion 4 of a thermally insulating layer comprising buildinginsulation material, and a second portion 5 of a thermally insulatinglayer, and a waterproof membrane 6. The autoclaved aerated concrete 15,which fills the gap between the roof elements 1 further comprises steelreinforcement 16. A binder 18 having a lower part 19 and a middle part20 is cast into the lower layer of concrete 3 is configured to stabilizethe upper layer of concrete sideways. There is an additional concreteelement 21 (poured in place) between two neighboring upper layers 2having an upper funnel shape portion 23 and a lower portion 22. Theportions 22 and 23 together form a hook-like shape that hooks around thelower corners of the upper layers 2, thereby preventing the concreteelement from being moved upwards in relation to the roof elements.

FIG. 7 shows the lower layer 3 of a load bearing roof element shaped asa rectangular container with five closed sides, and an open upper sidesuch that two open cavities are formed. The container-shaped lower layer3 has one inner wall 17 of concrete. A first layer of insulation willfill the two open cavities of roof element. During manufacture thislower layer 3 of concrete will actually be cast around a first layer ofinsulation, such that it is this first insulating layer that forms thecavities during manufacture.

FIG. 8a shows a first portion 4 of a thermally insulating layer andbinders 18 configured to stabilize the upper layer of concrete sideways.In FIG. 8b the first portion 4 of a thermally insulating layer has asecond layer of the first portion of a thermally insulating layercomprising a rigid and light material. In FIG. 8c there is a secondportion 5 of a thermally insulation layer comprising a rigid and lightmaterial configured to form a slope of the roof element. FIG. 8d showsthe roof element of FIGS. 8a-c having an upper layer of concrete 2 and alower layer of concrete 3 with the insulation layers (portions 4 and 5)and binders 18 of FIGS. 8a-c . The binders 18 prevent sideways movementof the upper layer 2 in relation to the lower layer 3.

FIG. 9 shows a cross section of a roof element 1 having an upper layerof concrete 2 and a lower layer of concrete 3, thermally insulatinglayer 4 and binders 18 in the form of steel rods.

FIG. 10 shows a further embodiment of a roof element 1 having an upperlayer of concrete 2 and a lower layer of concrete 3, a thermallyinsulating layer 4 and a binder 18 in the form a steel rod. In thisexample, the binder 18 has a lower part 19 which is fastened to thelower layer of concrete and an upper part 24 which is fastened to theupper layer of concrete 24.

FIG. 11 shows a further embodiment of a roof element 1 having an upperlayer of concrete 2 and a lower layer of concrete 3, a first portion ofthermally insulating layer 4, a second portion of insulating layer 5,and a binder 18. In this embodiment pairs of binders form the shape ofan X. The upper parts 24 of the binders are substantially horizontal.

FIG. 12 shows a further embodiment of a roof element 1 having an upperlayer of concrete 2 and a lower container-shaped layer of concrete 3(two internal containers in the longitudinal direction of the roofelement), a first portion of thermally insulating layer 4 having onelayer 4 a in the container and a second layer 4 b, and a second portionof insulating layer 5 comprising a rigid and light material such as PIR.

FIG. 13 shows a further embodiment of the presently disclosed precastload bearing roof element.

FIG. 14 shows a roof construction 11 having two roof elements 1, eachroof element having an upper layer of concrete 2 and a lower layer ofconcrete 3, a first portion of thermally insulating layer 4, a secondportion of insulating layer 5, and binders 18 to prevent sidewaysmovement of the upper layer 2 in relation to the lower layer 3. Thelower layer 3 is shaped as a rectangular container with five closedsides and an open upper side. In this example the container shaped lowerlayer 3 has one inner wall 17 of concrete in the longitudinal directionof the lower layer 3, dividing the container into two sub-containers.There is an additional concrete element 21 between two neighboring upperlayers 2. The gap between the lower layers 3 of two neighboring elementsis wedge-shaped, wherein a lower section of the wedge is filled withconcrete 15 and an upper section of the wedge is filled with a thermallyinsulating layer 25. The roof elements have a plurality of binders 18being cast into the lower layer 3 and the upper layer 2.

FIG. 15 shows a cross section of two neighboring roof elements 1 of aroof construction, the roof elements comprising brackets 26, preferablymade of steel, cast into the roof elements. The brackets of the two roofelements are bolted together.

FIG. 16 shows one embodiment of a roof element having brackets 26 forbolting the roof element to another roof element. The bracket can alsobe used for lifting the roof element in cables and positioning it on aload bearing construction.

FURTHER DETAILS OF INVENTION

The invention will now be described in further detail with reference tothe following items:

-   -   1. A precast load bearing roof element for a building comprising        an upper layer of concrete, a lower layer of concrete, and at        least one thermally insulating layer between the upper layer and        the lower layer, the roof element configured to be mounted on a        load bearing construction of the building.    -   2. The roof element according to any of the preceding items,        configured to be mounted horizontally, or configured to be        mounted such that the roof element is sloped less than 10°, or        less than 11°, or less than 12°, or less than 13°, or less than        14°, or less than 15°, or less than 20°, or less than 25° in        relation to a horizontal line.    -   3. The roof element according to any of the preceding items,        wherein the roof element forms a slab.    -   4. The roof element according to any of the preceding items,        suitable for a green roof.    -   5. The roof element according to any of the preceding items,        having a substantially plane shape.    -   6. The roof element according to any of the preceding items,        having a substantially plane upper surface of the upper layer.    -   7. The roof element according to any of the preceding items,        said roof element having a rectangular shape.    -   8. The roof element according to any of the preceding items,        wherein the at least one thermally insulating layer separates        the upper layer from the lower layer.    -   9. The roof element according to any of the preceding items,        wherein the at least one thermally insulating layer comprises at        least one first portion comprising building insulation material,        selected from the group of cellulose, glass wool, rock wool,        polystyrene, polyisocyanurate (PIR), polyurethane (PUR),        urethane foam, vermiculite, perlite, wood fiber, plant fiber,        recycled cotton denim, plant straw, and animal fiber.    -   10. The roof element according to any of the preceding items,        wherein the at least one thermally insulating layer is        configured to bear the weight of the upper layer.    -   11. The roof element according to any of the preceding items,        wherein the at least one thermally insulating layer comprises at        least one second portion on top of the first portion comprising        a rigid and light material, such as polyisocyanurate (PIR) and        polyurethane (PUR), the second portion configured to form a        slope of the upper layer in relation to the lower layer.    -   12. The roof element according to any of the preceding items,        further comprising at least one binder extending between the        lower layer and the upper layer.    -   13. The roof element according to item 12, wherein the at least        one binder is cast into the lower layer and upper layer.    -   14. The roof element according to any of items 12-13, the at        least one binder extending both in a vertical direction between        the upper layer and the lower layer and in a horizontal        direction, the at least one binder thereby being configured for        preventing sideways movement of the upper layer in relation to        the lower layer.    -   15. The roof element according to any of the preceding items,        wherein at least one of the upper layer and/or lower layer of        concrete is load bearing.    -   16. The roof element according to any of the preceding items,        wherein the lower layer is load bearing and shaped as a        rectangular container with five closed sides, and wherein the        upper side is open.    -   17. The roof element according to item 16, wherein the at least        one thermally insulating layer fills the inner volume of the        lower layer shaped as rectangular container.    -   18. The roof element according to any of the preceding items,        wherein the lower layer of concrete comprises one or more voids        or recesses filled with building insulation material.    -   19. The roof element according to any of the preceding items,        configured to be mounted on a load bearing column and beam        construction and/or load bearing walls.    -   20. The roof element according to item 19, wherein two opposite        ends of the roof element are configured to rest on two beams of        the column and beam construction or on the load bearing walls.    -   21. The roof element according to any of items 19-20, wherein        two opposite ends of the roof element are configured to rest on        L-shaped or inverted T-shaped beams of the column and beam        construction.    -   22. The roof element according to any of the preceding items,        wherein the thickness of the upper layer is between 50 mm and        250 mm.    -   23. The roof element according to any of the preceding items,        wherein the thickness of the lower layer is between 50 mm and        600 mm.    -   24. The roof element according to any of the preceding items,        wherein the thickness of the at least one thermally insulating        layer is between 50 mm and 300 mm.    -   25. The roof element according to any of the preceding items,        wherein the combined thickness of the upper layer, the lower        layer and the at least one thermally insulating layer is between        300 mm and 800 mm.    -   26. The roof element according to any of the preceding items,        wherein the length of the roof element is between 4 meters and        12 meters.    -   27. The roof element according to any of the preceding items,        wherein the width of the roof element is between 0.5 meters and        3 meters.    -   28. The roof element according to any of the preceding items,        further comprising a waterproof membrane attached on the upper        surface of the upper layer.    -   29. The roof element according to item 28, wherein the membrane        is welded on the upper surface.    -   30. The roof element according to any of items 28-29, wherein        the membrane is made of synthetic rubber and/or thermoplastic        and/or modified bitumen, and/or polyurethane and/or metal.    -   31. The roof element according to any of items 28-30, wherein        the membrane is seamless.    -   32. The roof element according to any of items 28-31, wherein        the membrane is configured to lead water towards a drain.    -   33. The roof element according to any of items 28-32, wherein        the membrane has an integral root repellant chemically        integrated in the membrane.    -   34. The roof element according to any of the preceding items,        further comprising a root repellent membrane.    -   35. The roof element according to any of the preceding items,        the upper layer and/or the lower layer and/or the thermally        insulating layer(s) further comprising reinforcing bars.    -   36. The roof element according to item 35, wherein the bars are        made of steel, polymers or alternate composite material.    -   37. The roof element according to any of items 35-36, comprising        reinforcing bars extending between the upper layer and the lower        layer.    -   38. The roof element according to any of items 35-37, wherein        the reinforcing bars are configured to bear at least a part of        the weight from the upper layer and from additional roof        installations.    -   39. The roof element according to any of the preceding items,        wherein the upper layer is slightly convex exteriorly and/or        sloped in relation to the lower layer.    -   40. The roof element according to item 39, wherein the height        difference between the highest point and the lowest point of the        outer surface of the convex upper layer is less than 200 mm.    -   41. A roof construction comprising a load bearing construction,        such as a column and beam construction, and at least two precast        load bearing roof elements according to any of items 1-40,        wherein the roof elements are positioned side by side forming a        gap between the roof elements, wherein the distance between the        roof elements is between 0 mm and 100 mm, or between 1 mm and        100 mm.    -   42. The roof construction according to item 41, wherein the gap        between the roof elements is filled with concrete and/or        autoclaved aerated concrete.    -   43. The roof construction according to item 42, wherein the        concrete in the gap is configured to resist diaphragm actions of        the roof elements.    -   44. The roof construction according to any of items 41-43,        further comprising additional waterproof membranes overlapping        two neighboring roof elements, thereby covering the gap between        the roof elements.    -   45. The roof construction according to item 44, wherein the        additional waterproof membrane is welded on the upper surfaces        of the two neighboring roof elements.    -   46. The roof construction according to any of items 44-45,        wherein the width of the additional waterproof membrane is        between 30 mm and 900 mm.    -   47. The roof construction according to any of items 41-46,        further comprising additional insulation elements on the upper        side of the upper layer, wherein the additional insulation        elements cover at least a part of the vertical extensions of the        sections of concrete and/or autoclaved aerated concrete.    -   48. The roof construction according to item 47, wherein the        additional insulation elements are mounted at the shorter edges        of the rectangular shaped roof element, and wherein the        additional insulation elements extend over at least two roof        elements.    -   49. The roof construction according any of items 47-48, wherein        the thickness of the additional insulation elements is between        50 mm and 200 mm.    -   50. The roof construction according any of items 47-49, wherein        the cross section of the additional insulation elements is        substantially triangular.    -   51. The roof construction according to any of items 47-50,        wherein the sides of the additional insulation elements fronting        the horizontal center of the roof elements are sloped.    -   52. The roof construction according to any of items 47-51,        further comprising a layer of metal, such as steel, covering the        additional insulation elements.    -   53. The roof construction according to item 52, the insulation        elements having a cover, wherein the layer of metal is        positioned on the cover of the additional insulation elements.    -   54. A method for manufacturing a load bearing roof element,        comprising the steps:        -   casting a lower layer of concrete;        -   adding a layer of thermally insulating material on the lower            layer of concrete;        -   casting an upper layer of concrete on the layer of thermally            insulating material;        -   attaching a waterproof membrane on the upper surface of the            upper layer of concrete, such that the membrane covers at            least a part of the entire upper surface of the roof            element.    -   55. The method according to item 54, wherein the membrane is        welded on the upper surface of the upper layer of concrete.    -   56. The method according to any of items 54-55 wherein the roof        element is a roof element according to any of items 1-40.    -   57. A method for installing a roof construction on a load        bearing construction of a building, comprising the steps:        -   manufacturing a number of thermally insulated load bearing            roof elements according to the method of any of items 54-56;        -   lifting the thermally insulated load bearing roof elements            and placing them on the load bearing construction side by            side, thereby forming gaps between the roof elements,            wherein the distances between the roof elements are between            0 mm and 100 mm, or between 1 mm and 100 mm;        -   filling the gaps with concrete and/or autoclaved aerated            concrete.    -   58. The method according to item 57, further comprising the step        of welding additional waterproof membranes overlapping two        neighboring roof elements, thereby covering the gap between the        roof elements.    -   59. The method according to any of items 57-58, wherein the roof        elements are roof elements according to any of items 1-40.

1-30. (canceled)
 31. A precast load bearing roof element for a greenroof of a building, comprising: a load distributing concrete upper floorlayer; a load bearing steel reinforced lower deck layer of concrete; athermally insulating layer located between and separating the upperfloor layer and the lower deck layer; and a plurality of independentbinders extending between the lower deck layer and the upper floorlayer; the roof element configured to be mounted on and span a loadbearing construction of the building.
 32. The roof element according toclaim 31, wherein the thermally insulating layer substantially fills aninner volume formed between the upper floor layer and the lower decklayer.
 33. The roof element according to claim 31, wherein the thermallyinsulating layer forms a non-ventilated inner volume between the upperfloor layer and the lower deck layer.
 34. The roof element according toclaim 31, wherein the lower deck layer of concrete has embedded,substantially horizontal reinforcement steel bars.
 35. The roof elementaccording to claim 34, wherein the substantially horizontalreinforcement steel bars are placed at least 40 mm from an underside ofthe lower deck layer.
 36. The roof element according to claim 31,wherein the roof element is configured such that the upper floor layerof concrete can bear a weight of at least 500 kg/m2.
 37. The roofelement according to claim 31, wherein thermal conduction and/or athermal bridge between the upper floor layer and the lower deck layer ofconcrete is limited to the binders.
 38. The roof element according toclaim 31, wherein the independent binders are arranged not to carry anyweight of the layers.
 39. The roof element according to claim 31,wherein thermal conduction and/or a thermal bridge between the upperfloor layer and the lower deck layer of concrete is substantiallynegligible.
 40. The roof element according to claim 31, wherein theplurality of binders extend through the thermally insulating layer. 41.The roof element according to claim 31, wherein the plurality of bindersare cast into the lower deck layer and cast into the upper floor layer.42. The roof element according to claim 31, wherein the plurality ofbinders extend both in a vertical direction and in a horizontaldirection between the upper floor layer and the lower deck layer ofconcrete.
 43. The roof element according to claim 31, wherein theplurality of binders are configured for preventing sideways movement ofthe upper floor layer of concrete in relation to the lower deck layer ofconcrete.
 44. The roof element according to claim 31, wherein theplurality of binders are configured to prevent sideways movement only ofthe upper floor layer of concrete in relation to the lower deck layer ofconcrete.
 45. The roof element according claim 31, wherein the pluralityof binders are steel rods, stainless steel rods or galvanized steelrods.
 46. The roof element according to claim 31, wherein the thermallyinsulating layer is configured to bear the weight of the upper floorlayer of concrete.
 47. The roof element according to claim 31, furthercomprising a waterproof membrane attached on an upper surface of theupper floor layer, the membrane covering the entire upper surface of theupper floor layer, wherein the membrane is welded on the upper surface.48. A roof construction comprising a load bearing construction, and atleast two precast load bearing roof elements according to claim 31,wherein each roof element is spanning a load bearing construction andwherein the roof elements are positioned side by side forming a gapbetween the roof elements, wherein the distance between the roofelements is between 0 mm and 100 mm, or between 1 mm and 100 mm.
 49. Theroof construction according to claim 48, further comprising additionalwaterproof membranes overlapping two neighboring roof elements, therebycovering the gap between the roof elements, wherein the additionalwaterproof membrane is welded on upper surfaces of the two neighboringroof elements, and wherein the roof construction further comprisesadditional insulation elements on the upper side of the upper floorlayer, wherein the additional insulation elements cover at least a partof a vertical extensions of the sections of concrete and/or autoclavedaerated concrete.
 50. A method for manufacturing a load bearing roofelement, comprising the steps: casting a lower deck layer of concretearound an inner volume of insulation and around reinforcing steel bars;casting a lower end of a plurality of independent binders into an uppersurface of the lower deck layer of concrete; adding a layer of thermallyinsulating material on the lower deck layer of concrete; casting anupper floor layer of concrete on the layer of thermally insulatingmaterial and casting an upper end of said binders into a lower surfaceof the upper floor layer of concrete; and attaching a waterproofmembrane on an upper surface of the upper floor layer of concrete, suchthat the membrane covers at least the entire upper floor surface of theroof element, wherein the membrane is welded on the upper surface of theupper floor layer of concrete.